1. Field of the Invention
This invention relates to the art of papermaking, particularly to treating a bleached kraft paper product with pressure and heat to improve its wet strength while preserving its folding endurance.
2. Description of the Prior Art
The kraft process is a method of preparation of an aqueous slurry of fibers by treatment of a suitable renewable raw material. In most pulping process, a considerable portion of the natural lignin in wood, grass or other vegetative matter is rendered soluble by chemical reaction with one or more nucleophilic reagents. In the kraft process, the nucleophilic reagents are sulfide and hydroxide ions, which are used highly alkaline conditions. Variations of the kraft process include the earlier practiced soda process, using hydroxyl ions derived from metals in Group IA of the periodic table, namely lithium, sodium, potassium, rubidinium and cesium. A second variation involves the use of anthraguinone (AQ) or substituted anthraquinones as additional nucleophiles. Anthraquinone can be used in the soda process, in which case the process is known as the soda-AQ process, or in the kraft process which is then known as the kraft-AQ process. Such variations in the kraft process are well known in the industry and pulps prepared by any of these variations can be used in practicing the present invention.
If desired, the soda-AQ, kraft and kraft AQ pulps can be rendered white by application of suitable bleaching agents. Such agents are usually electrophilic in nature and may include chlorine, chlorine dioxide, sodium hypochlorite, hydrogen peroxide, sodium chlorite, oxygen and ozone. Use is often in sequential stages and a suitable nucleophilic agent, customarily hydroxyl ion, may be used in intermediate stages. "Kraft paper" is paper made from pulp produced by the kraft process. Bleached kraft paper, because of its low lignin content, has low wet strength; hence it is desirable to develop this quality of bleached kraft products.
In the art of making kraft paper products, it is conventional to subject felted fibers to wet pressing to unite the fibers into a coherent sheet. Pressure is typically applied to a continuous running web of paper by a series of nip rolls which, by compressing the sheet, both increase its volumetric density and reduce its water content. The accompanying FIG. 1 shows in simplified diagrammatic form a typical papermaking machine, including a web former and three representative pairs of wet press rolls. Also shown are drying rolls whose purpose is to dry the paper to a desired final moisture content, and a calendar stack to produce a smooth finish. At least some of the rolls are ordinarily heated to hasten drying. (The drawing is simplified--there are many more drying rolls in actual practice.)
There is currently considerable interest in treatments involving heat and pressure, or heat alone, during or after the production process, to improve various qualities of paper products. Qantifiable paper qualities include dry tensile strength, wet tensile strength, reverse folding endurance, compressive strength and stiffness, among others. Which qualities should desirably be enhanced depends upon the intended application of the product. For paper to be used in humid or wet environments, two qualities of particular interest are wet strength and folding endurance, both of which can be measured by well-known standard tests. As used herein, then, "wet strength" means wet tensile strength as measured by American Society for Testing and Materials (ASTM) Standard D829-48. "Folding endurance" is defined as the number of times a board can be folded in two directions without breaking, under conditions specified in Standard D2176-69. "Basis weight" is the weight per unit area of the dried end product.
Prior workers in this field have recognized that high-temperature treatment of linerboard can improve its wet strength. See, for example E. Back, "Wet stiffness by heat treatment of the running web", Pulp & Paper Canada, vol. 77, No. 12, pp. 97-106 (Dec. 1976). This increase has been attributed to the development and cross-linking of naturally occurring polysaccharides and other polymers, which phenomenon may be sufficient to preserve product wet strength even where conventional synthetic formaldehyde resins or other binders are entirely omitted.
It is important to note that wet strength improvement by heat curing has previously been thought attainable only at the price of increased brittleness (i.e., reduced folding endurance). Therefore, most prior high-temperature treatments have been performed on particle board, wallboard, and other products not to be subjected to flexure. The known processes, if applied to bleached kraft paper, would produce a brittle product. Embrittled paper is not acceptable for many applications involving subsequent deformation, and therefore heat treatment alone, to develop wet strength of bleached kraft products, has no gained widespread acceptance. As Dr. Back has pointed out in the article cited above, "The heat treatment conditions must be selected to balance the desirable increase in wet stiffness against the simultaneous embrittlement in dry climates." Significantly, in U.S. Pat. No. 3,875,680, Dr. Back has disclosed a process for heat treating already manufactured corrugated board to set previously placed resins, the specific purpose being to avoid running embrittled material through a corrugator. It is plain that added wet strength and improved folding endurance were previously thought incompatible results.
It is therefore an object of the invention to produce bleached kraft paper products having both greatly improved wet strength and good folding endurance. Another goal is to achieve that objective without resorting to synthetic resins or other added binders and wet strength agents.
With a view to the foregoing, a process has been developed which dramatically and unexpectedly increases not only the wet strength of bleached kraft paper, but also preserves its folding endurance. In its broadest sense, the invention comprises steps of (1) subjecting paper produced from bleached kraft pulp to high pressure densification, and (2) heating the board to an internal temperature of at least 420.degree. F. (216.degree. C.) for a period of time sufficient to increase the wet strength thereof. This method produces a product having folding endurance greatly exceeding that of similar paper whose wet strength has been increased by heat alone. This is clearly shown by our tests exemplified below.
While the tests set out in Examples 1-2 have carried out the invention in a static press, it is preferred that the heat and pressure be applied to continuously running paper by hot pressure rolls, inasmuch as much higher production rates can be attained.
We prefer to raise the internal temperature of the paper to at least 465.degree. F. (240.degree. C.), as greater wet strength is then achieved. This may be because at higher temperatures, shorter step duration is necessary to develop bonding, and there is consequently less time for fiber degradation to occur. Also, shorter durations enable one to achieve higher production speeds.
It should be noted that the heating rate, and thus the required heating duration at a particular temperature, depends on the method of heat transfer chosen. Furthermore, it is desirable to raise the web temperature as rapidly as possible to the chosen treating temperature. Improved heating rates can be achieved by using high roll temperatures and/or by applying high nip forces to the press roll against the sheet on the hot rolls. That high pressure dramatically improves heat transfer rates has previously been disclosed. One worker has attributed this to the prevention of vapor formation at the web-roll interface.
While the invention may be practiced over a range of temperatures, pressures and durations, these factors are interrelated. For example, the use of higher temperatures requires a heating step of shorter duration, and vice-versa. At 465.degree. F., a duration of 60 seconds has been found sufficient to obtain the desired improvements, while at 420.degree. F., considerably longer time is required.
It is presently preferred that, for safety reasons, the roll temperature be not greater than the web ignition temperature (572.degree. F., 300.degree. C.); however, even higher roll temperatures may be used if suitable precautions, such as the provision of an inert atmosphere, or rapid removal of paper from the hot environment, are taken.